Check - accurately - the size!
I had 2 M-audio keyboards, and wanted to add a third; the Behringer UMX looked better (and, although it's very subjective, I think it is a little better), and ordered one. I was not at all prepared to find that it was almost 1/4 inch longer than the M-audio. (So now there are 3 UMX's!)
I envy you the 2 CMKs that you have - on my wish list!. But I'd be concerned that the difference in touch between them and the less expensive variety would drive me nuts. But just having a 3rd manual, if you're willing to deal with the mis-match, is good too. Just be aware that you are compromising, and only you can evaluate its worth to you.
Rick